The present invention relates to a digital communication system of the type using error correction codes and, more particularly, to a digital communcation system which includes an improved scrambler/descrambler.
In a digital communication system of the type described, a transmitter station passes an input data signal through a scrambler to randomize it. Then, the random signal passes through an encoder to error-correction encode the scrambler output. The encoder output is sent to a receiver station via a transmission path. The receiver station, in turn, routes the received data signal through a decoder to error-correction decode it. Then, it passes through a descrambler to randomize the decoder output to thereby recover the original data signal.
As will be described later in detail, the descrambler comprises a shift register, with an inhibit circuit connected to intermediate taps of the shift register. An Exclusive-OR gate (modulo 2 adder) is connected to intermediate taps of the shift register and to an output of the inhibit circuit. The incoming data signal is applied to the shift register by way of the Exclusive-OR gate. Also, the intermediate tap output of the shift register is fed back to the shift register via the Exclusive-OR gate. The problem encountered with such a construction is that data errors developed in the transmission path are magnified by the descrambler at the receiver station.